Effort has been made in the past, without success, to provide the electronic industry with a glass material for sealing semiconductor ceramic packages at the lowest possible temperature and with sufficient mechanical strength to maintain hermeticity during thermal shocks and other conditions as specified by the MIL-STD-883 specifications.
The technical requirements of an adequate sealing glass material are severe. The material problem has arisen particularly with the development of the dual-in-line type ceramic packaging technology and its wide acceptance as a relatively inexpensive multilead hermetic package for integrated semiconductor circuits. The glass seal must not only bond two ceramic parts together but must also provide a strong hermetic seal with a substantially mismatched expansion metal lead frame within the glass layer.
Similarly, in the formation of cathode ray tubes, in order not to damage the heat sensitive phosphor coating, metallic film shield or electrical contact, glass parts must be sealed at the lowest possible temperature.
Heretofore successful sealing materials for the above applications have been made with glasses, known generically as solder glasses, containing mainly lead, zinc and boron oxides, mixed as a powder with an inert, low expansion ceramic powder such as beta-eucryptite, fused silica, or zirconium silicate to modify the internal structure of the glass once the seal has been formed by heat induced recrystallization. The presence of a multitude of crystals within the glass layer, plus the presence of a low expansion ceramic filler, prevents the propagation of surface cracks through the glass-polycrystalline seal which is subjected to considerable tensile stresses. The sealing temperatures of these materials are commonly too high to be used with many semiconductor devices, namely, those known in the semiconductor industry as MOS (Metal Oxide Silicon), LIC (Linear Integrated Circuits) and CCD (Charge Coupled Devices) integrated circuits, which are surface sensitive and prone to failure when heated over about 430.degree. C.